The invention relates to an improved process for producing polyether polyols in the presence of double-metal cyanide (xe2x80x9cDMCxe2x80x9d) catalysts by polyaddition of alkylene oxides onto initiator compounds comprising active hydrogen atoms.
The production of polyether polyols is typically carried out industrially by polyaddition of alkylene oxides onto polyfunctional initiator compounds such as, for example, alcohols, acids or amines by means of base catalysis (e.g. KOH) (see, for example, Gum, Riese and Ulrich (Editors): xe2x80x98Reaction Polymersxe2x80x99, HanserVerlag, Munich, 1992, pp 75-96). After the polyaddition has finished, the basic catalyst has to be removed from the polyether polyol in a very elaborate process, e.g. by neutralization, distillation and filtration. The polyether polyols produced by a base-catalyzed method have the disadvantage, in addition, that with increasing chain-length the content of monofunctional polyethers with terminal double bonds (so-called xe2x80x9cmono-olsxe2x80x9d) rises steadily and functionality falls.
The polyether polyols that are obtained can be employed for the production of polyurethanes (e.g. elastomers, foams, coatings), particularly for the production of polyurethane flexible foam materials. Flexible foam materials offer slight resistance to compressive stress, are open-celled, air-permeable and reversibly deformable. A distinction is made between slabstock foams and molded foams (see, for example, Kunststoffhandbuch, Vol. 7, 3rd Edition, Hanser Verlag, Munich, 1993, pp 193-252). Slabstock foam materials are produced continuously or discontinuously as semifinished products and are subsequently tailored to the dimension and contour corresponding to the application (e.g. upholstered furniture, mattresses). Molded foams, on the other hand, are produced by a discontinuous process in which the foam bodies are obtained directly in the desired shape (by foam expansion to fill out an appropriate mold).
DMC catalysts for the production of polyether are known. (See, for example, U.S. Pat. Nos. 3,404,109, 3,829,505, 3,941,849 and 5,158,922). The use of these DMC catalysts for the production of polyether polyols brings about a reduction in the proportion of monofunctional polyethers (mono-ols) in comparison with the conventional production of polyether polyols with basic catalysts. Improved DMC catalysts, such as are described in EP-A 700 949, EP-A 761 708, WO 97/40086, WO 98/16310, DE-A 197 45 120, DE-A 197 57 574 or DE-A 198 102 269, for example, additionally possess extraordinarily high activity and enable the production of polyether polyols at very low catalyst concentration (25 ppm or less), making separation of the catalyst from the polyol unnecessary.
In the course of producing polyurethane foams, in particular, polyurethane flexible foam materials, the polyether polyols that are obtained by DMC catalysis can lead to application problems, for example, destabilization of the foam (increased susceptibility to collapse) or increased coarse-cell structure. DMC-catalyzed polyether polyols, therefore, cannot replace corresponding base-catalyzed polyols in polyurethane flexible-foam applications in all cases without adaptation of the formulation.
It has now been found that polyether polyols that are produced completely or partially with DMC catalysis possess distinctly improved foaming properties in the course of the production of polyurethane foams if the polyether polyol is conducted through a suitable mixing unit during or after the DMC-catalyzed polyaddition of alkylene oxides onto initiator compounds comprising active hydrogen atoms.
The present invention relates to an improved process for producing polyether polyols, wherein the polyether polyol is produced completely or partially by DMC-catalyzed polyaddition of alkylene oxides onto initiator compounds comprising active hydrogen atoms and wherein the polyether polyol is conducted through a zone with high energy density during or after the DMC-catalyzed polyaddition. The present invention also relates to the use of the polyether polyols obtained in this way for the purpose of producing polyurethane foam, in particular polyurethane flexible foam materials.
The DMC catalysts suitable for the process according to the invention are known. See, for example, JP-A 04-145123, EP-A 654 302, EP-A 700 949, EP-A 743 093, EP-A 761 708, WO 97/40086, WO 98/16310, WO 99/19062, WO 99/19063, WO 99/33562, DE-A 198 34 572, 198 34 573,198 42 382, 198 42 383,199 05 611, 199 06 985,199 13 260, 199 20 937 or 199 24 672. A typical example is the highly active DMC catalyst described in EP-A 700 949, which, in addition to a DMC compound (e.g. zinc hexacyanocobaltate(III)) and an organic complex ligand (e.g. tert. butanol), also contains a polyether polyol with a number-average molecular weight greater then 500 g/mol.
Compounds with molecular weights from 18 to 2,000 g/mol, preferably, 62 to 1,000 g/mol, and 1 to 8, preferably, 2 to 6, hydroxyl groups are utilized as the initiator compounds having active hydrogen atoms. Examples of such initiator compounds useful in the present invention include butanol, ethylene glycol, diethylene glycol, triethylene glycol, 1,2-propylene glycol, 1,4-butanediol, 1,6-hexanediol, bisphenol A, trimethylolpropane, glycerin, pentaerythritol, sorbitol, raw sugar, degraded starch, water or so-called pre-lengthened initiators.
Alkylene oxides useful in the present invention include ethylene oxide, propylene oxide, butylene oxide and mixtures thereof. The synthesis of the polyether chains can be carried out with only one monomeric epoxide or randomly or blockwise with 2 or 3 different monomeric epoxides. Further details can be gathered from xe2x80x98Ullmanns Encyclopxc3xa4die der industriellen Chemiexe2x80x99, Volume A21, 1992, 670 et. seq.
The polyaddition can be carried out by any alkoxylation process that is known for DMC catalysis.
For example, a conventional batch process can be employed. In this case, the DMC catalyst and the initiator compound are fed to the batch reactor, then the reactor is heated up to the desired temperature and a quantity of alkylene oxide sufficient for activating the catalyst is added. As soon as the catalyst has been activated, which becomes noticeable, for example, by a drop in pressure in the reactor, the remaining alkylene oxide is continuously added in metered amounts until the desired molecular weight of the polyether polyol is attained.
A continuous process may also be employed in which a pre-activated mixture composed of DMC catalyst and initiator compound is continuously supplied to a continuous reactor, e.g. to a continuous stirred-tank reactor (xe2x80x9cCSTRxe2x80x9d) or a tubular reactor. Alkylene oxide is metered into the reactor, and the product is drawn off continuously.
In preferred embodiment of the present invention, DMC-catalyzed polyaddition is carried out in accordance with a process in which the initiator compound is added continuously in metered amounts during the polyaddition. In this regard, the DMC-catalyzed polyaddition with continuous metering of initiator can be effected by a batch process such as is taught by WO 97/29146, or by a continuous process, such as that disclosed in WO 98/03571.
The DMC-catalyzed polyaddition can be effected at pressures from 0.0001 to 20 bar, preferably, from 0.5 to 10 bar, more preferably, from 1 to 6 bar. The reaction temperatures are from to 20 to 200xc2x0 C., preferably, 60 to 180xc2x0 C., more preferably, 80 to 160xc2x0 C.
The DMC catalyst concentration is generally from 0.0005 to 1 wt. %, preferably, 0.001 to 0.1 wt. %, more preferably, 0.001 to 0.01 wt. %, relative to the quantity of polyether polyol to be produced.
In accordance with the invention, the polyether polyol is conducted, during or after the DMC-catalyzed polyaddition, through a zone with high energy density, such as arises in a suitable mixing unit. The basic structure of suitable mixing units for the treatment according to the invention of the polyether polyols is described below.
Suitable mixing units are distinguished by the fact that, by reason of their geometry, they introduce a high local energy density into the product in the form of energy of flow. Since high pressures are frequently employed for this task, these mixing units are also designated as high-pressure homogenizers. Mixing units that are particularly suitable for such tasks are static mixers and/or nozzle units. Particularly suited are simple perforated screens, flat nozzles, jagged nozzles, knife-edge nozzles, microfluidizers, such as are described in U.S. Pat. No. 4,533,254 (xe2x80x9cthe ""254 patentxe2x80x9d) which is incorporated herein by reference, microstructure mixers, microstructure components or jet dispersers. Further geometries that operate according to the same principle of these or other nozzle units are readily available to a person skilled in the art. The functional principle of these nozzle units will be explained on the basis of the example represented by a simple perforated screen. The stream of product is pressurized by a pump and expanded through the orifice. By reason of the sudden constriction of cross-section, the stream of product in the nozzle is greatly accelerated. Depending on the geometry of the orifice, two different kinds of force may act on the product in this process. Either the stream of product is accelerated so much that the flow in the nozzle is turbulent, or, in the case of a laminar flow, a so-called extensional flow forms in the nozzle.